1. Field of the Invention
The present invention relates to a phosphorescent compound and an organic electroluminescent device (OELD) and more particularly to a phosphorescent compound having high triplet energy and an OELD using the same.
2. Discussion of the Related Art
An OELD emits light by injecting electrons from a cathode as an electron injection electrode and holes from an anode as a hole injection electrode into an emission compound layer, combining the electrons with the holes, generating an exciton, and transforming the exciton from an excited state to a ground state. A flexible substrate, for example, a plastic substrate, can be used as a base substrate where elements are formed. Since the OELD does not require a backlight assembly, the OELD has low weight and low power consumption. Moreover, the OELD can be operated at a voltage (e.g., 10V or below) lower than a voltage required to operate other display devices.
Dopant is added into a host of the emission layer. For example, a red emission layer includes a host of 4,4′-N,N′-dicarbazolbiphenyl (CBP) of about 30 nm and a dopant of bis(2-phenylquinoline)(aceteylacetonate)iridium(III) (Ir(phq)2acac). The dopant is added by a weight % of about 5 to 10.
Recently, a phosphorescent compound is more widely used for the emission layer than a fluorescent compound. The fluorescent compound only uses singlet energy corresponding to about 25% of excitons for emitting light, and triplet energy corresponding to about 75% of excitons is lost as a heat. However, the phosphorescent compound uses not only the singlet energy but also the triplet energy for emitting light. The phosphorescent dopant includes a heavy atom, such as iridium (Ir), platinum (Pt) and europium (Eu) at a center of an organic compound and has a high electron transition probability from the triplet state to the single state.
However, since an emitting yield of the dopant is rapidly reduced because of a concentration quenching, the dopant cannot comprise the emission layer for itself. Accordingly, a host with the dopant is used for the emission layer.
In the OELD, a hole from the anode and an electron from the cathode are combined in the host of the emission layer such that a singlet state exciton and a triplet state exciton are generated. The singlet state exciton is transited into a singlet energy or a triplet energy of the dopant. The triplet state exciton is transited into a triplet energy of the dopant. Since the exciton transited into the singlet energy of the dopant is re-transited into the triplet energy of the dopant, a destination of all exciton is a triplet energy level of the dopant. The exciton at the triplet energy level of the dopant is transited into a ground state to emit light.
For an efficient energy transition into the dopant, a triplet energy of the host should be larger than that of the dopant. However, referring to FIG. 1, CBP, which is widely used for the host, has a triplet energy of about 2.6 eV, while a phosphorescent dopant, for example, iridium-bis-(4,6-difluorophenylpyridinato-N—C2)-picolinate (FIrpic), has a triplet energy larger than 2.6 eV. Accordingly, an energy counter-transition from the dopant to the host is generated such that an emission yield is reduced. Particularly, the emission yield reduction is strongly generated in a low temperature. To prevent these problems, a new phosphorescent compound having a triplet energy above 2.6 eV and having a thermal stability is required.
In addition, when a triplet energy of the hole transporting layer and the electron transporting layer, which are adjacent to the emission layer, is smaller than that of the dopant, an energy counter-transition from the host or dopant to the hole transporting layer and the electron transporting layer is generated such that an emission yield is also reduced. Accordingly, a triplet energy is an important fact in the hole and electron transporting layers as well as the host.